The present invention relates to a method of detecting a slip of a driving wheel of an automotive vehicle which is likely to occur at start-up or acceleration.
A slip is detected to generate a slip indicative signal to be used in controlling a power train such that the occurrence of a slip is prevented upon detection of the slip. According to a conventional method of controlling a power train so as to prevent the slip from occurring, as disclosed in Laid-open Japanese Patent Application No. 58-38347, a slip is detected on a difference between an average value between revolution speeds of two front wheels and that between revolution speeds of two rear wheels (driving wheels), and the supply of fuel to an engine, which is a power source of the power train, is discontinued when the slip is detected in order to prevent the slip from occurring.
However, this slip detecting method poses a problem which attributes to the fact that each of the four wheels needs one wheel revolution speed sensor, thus requiring four in total per each automotive vehicle. The problem is that a detector carrying out the method is complicated in installing including wiring of the sensors and requires an increasing number of components, thus pushing up cost and accounting for high rate of failures of the detector.
Referring to FIG. 11, there is shown how a driving force of a driving wheel varies versus a variation in a slip rate S of the driving wheel for different road conditions including a dry road, a wet road and a snow road. The slip rate S is expressed as EQU S=(PSD-GSV)/PSD
where:
PSD is an abbreviation of a peripheral speed of a driving wheel, PA0 GSV is an abbreviation of a ground speed of a vehicle.
When a driving wheel is given a torque greater than a value corresponding to a driving force plotted on the corresponding curve to one of the different road conditions, the driving wheel begins to slip, and once it slips, the slip rate increases, whereas the driving force decreases, and the driving force slips at a rapid rate owing to the feedback between the slip rate which increases and the driving force which decreases under the application of output torque by the power train, thus disabling the vehicle to start and causing the vehicle to side slip. Therefore, it is desired that the power train be controlled such that the slip rate S always stays lower than a predetermined value which is determined so as to prevent the output torque from exceeding the plotted driving force level for each of the road condition. By so controlling, the vehicle is allowed to perform safe start-up and acceleration even on wet roads or snowy roads.